In high frequency communication devices that are equipped with a processing circuit for processing an input signal and outputting a resulting signal, in general, the processing circuit is housed in a metal case to prevent influence of external electromagnetic waves on a signal being processed and leakage of electromagnetic waves to the outside from the high frequency communication device. In this case, there is a problem that propagation and resonance of electromagnetic waves in the case cause coupling (electromagnetic coupling) between an input signal and an output signal. The metal material of the walls of the case is a factor in accelerating such coupling. This problem is now particularly remarkable because of increase of communication frequencies.
In high frequency communication devices, it is known to provide an internal surface of a case with a sheet-like electromagnetic wave absorbing body as a means of reducing the degree of coupling between signals in circuits such as an input signal and an output signal. An example of electromagnetic wave absorbing bodies of that kind is one in which particles including a soft magnetic material are dispersed in a matrix including a resin material, or the like.
For example, Patent Document 1 discloses a technique that is employed in a high frequency amplifier in which an amplification device is housed in a metal case and lead wires are connected to the input side and the output side of the amplification device. Electromagnetic coupling between the input side and the output side is prevented by providing an electromagnetic wave absorbing body inside and adjacent to at least one of the pair of side walls, the ceiling, and the pair of end walls of the case. Patent Document 1 discloses, as an example of the electromagnetic wave absorbing body, one obtained by mixing an Fe-7Cr-9Al powder made of a soft metal into polyethylene chloride rubber at 15 to 45 vol % and rolling a resulting mixture into a sheet having a thickness of 0.5 to 1.5 mm.
Patent Document 1: JP-A-2002-164687